Remark
You are using the midpoint formula. Instead of finding the midpoint, you are looking for one of the points, so you have to rearrange the formula a little bit.
Givens
Midpoint (4,2)
One endpoint (6,1)
Object
Find the other endpoint.
Formula
m(x,y) = (x1 + x2)/2, (y1 + y2)/2)
Solution
Find the x value
4 = (6 + x2)/2 Multiply both sides by 2
4*2 = 6 + x2 Subtract 6 from both sides.
8 - 6 = x2
x2 = 2
Find the y value
2 = (1 + y2)/2 Multiply by 2
4 = 1 + y2 Subtract 1 from both sides.
4 - 1 = y2
y2 = 3
Conclusion
R(x,y) = (2,3)
